The function of skeletal muscle is to produce the contractile force necessary for movement. One of the key proteins involved in muscle contraction is myosin, a hexamer consisting of two heavy chains and four light chains. Myosin heavy chain (MyHC) contains the motor domain and the rod domain necessary for thick filament formation. There are 8 known isoforms of MyHC expressed in striated muscle, 2 developmental and 6 adult. While the function and expression of the adult isoforms has been extensively characterized, relatively little is known about the role of the embryonic and perinatal skeletal muscle isoforms or of the factors regulating their expression. Their expression is initiated at mid-gestation, reaches a peak around birth, and is rapidly down regulated during early postnatal growth as they are replaced by the adult MyHC isoforms. During the prenatal period, muscle contraction but no coordinated movement occurs. Thus the exact function of these isoforms is poorly defined. We propose to evaluate the regulation and function of the two developmental MyHC isoforms using molecular and genetic approaches. First, we propose to study the cis- and trans-regulatory factors governing embryonic and perinatal MyHC expression by functionally analyzing the upstream regulatory regions of both genes. Promoter activities will be tested in cell culture and in mice; in the latter, both plasmid DNA injection and transient transgenics will be used to identify elements necessary for regulating the magnitude and pattern of expression during muscle development. Second, we will use homologous recombination to create mice in which either the embryonic or perinatal MyHC gene has been rendered null and study the resulting phenotype. Finally, to determine the role of the motor domain of embryonic MyHC in muscle development, we will create transgenic mice harboring a dominant mutation in the ATP binding domain. Specifically, we will test the hypothesis that embryonic MyHC contractile function is necessary for muscle development. These studies will define the role of the developmental MyHC isoforms in skeletal muscle form and function.